Boko Haram militants ambushed a military convoy in northeastern Nigeria, killing more than a dozen military and police personnel, the army said on Wednesday, December 26.

But an officer, speaking on condition of anonymity, said the militants had overrun an army base, and put the death toll at 18.

An army statement said the attackers ambushed a military convoy late Monday on a highway linking Maiduguri, the capital of Borno state, with Damaturu in neighboring Yobe state.

“Sadly, 13 military and a police personnel … paid the supreme price while extricating themselves from the ambush,” said the statement, signed by military spokesperson Colonel Onyema Nwachukwu.

The attack followed an attempted raid on a nearby military base along the highway in Kukareta village, 20 km (12 miles) from Damaturu, Nwachukwu said.

That attack was “repelled” by troops who “engaged and overpowered the terrorists, killing several of them,” Nwachukwu said, adding that a soldier was injured.

Other sources blame Islamic State West Africa Province, put death toll at 18

Other security sources said fighters traveling in eight technicals – trucks fitted with anti-aircraft guns – had overrun the base.

“The terrorists killed 17 soldiers in the attack, which lasted for more than an hour,” an officer told AFP on condition of anonymity.

“They took over the military base after overpowering troops and burnt it down. A policeman was killed when their vehicle which was heading to Damaturu ran into crossfire but the driver escaped unhurt,” the military source said.

The assailants were from the Islamic State West Africa Province faction of Boko Haram, the sources said.

A medic in Damaturu hospital said 18 bodies were taken to the morgue late on Monday. A civilian militiaman in the area gave a similar account and said the fighters withdrew after torching the base.

“They burnt the base and two military vehicles. They also burnt two primary schools in Kukareta and neighboring Ngaurawa village,” said the militiaman.

The 120-km (75-mile) Maiduguri-Damaturu highway has seen repeated attacks, including raids on military bases and markets.

In the worst attack targeting motorists, Boko Haram fighters disguised as soldiers blocked the highway and killed 167 people in September 2013.

Surge in Boko Haram attacks

Boko Haram split into two factions in mid-2016 over ideological differences. One is led by Abu Mus’ab Al-Barnawi and largely focuses on attacking military and government targets, while the other, led by Abubakar Shekau, is notorious for suicide bombings and indiscriminate killings of civilians.

Shekau has pledged allegiance to ISIS leader Abu Bakr Al-Baghdadi, but ISIS central gave its formal backing to the Barnawi faction, which is known as Islamic State West Africa Province.

ISWAP has lately intensified its armed campaign, launching a number of major assaults on military targets in Borno and neighboring Yobe state amid signs of a takeover by more hardline leaders.

There have been dozens of attacks on military bases since July. Most of the attacks have been blamed on ISWAP, or claimed by ISIS as ISWAP attacks. There has been an upsurge in attacks by both factions in recent weeks.

The military on November 30 lashed out at the media, saying some media outlets were “creating erroneous impression of the Nigerian Army through inaccurate and false publication of casualty figures.”

The military has even threatened legal action against organisations publishing unofficial figures.

Borno and Yobe states, along with nearby Adamawa state, have born the brunt of nine years of jihadist violence that has claimed 27,000 lives and forced 1.8 million people to flee their homes. The conflict has also spilled over into Nigeria’s northern neighbours Chad, Niger and Cameroon.

The recent surge in Boko Haram attacks has increased pressure on President Muhammadu Buhari, who was elected in 2015 on a promise to defeat the Islamists and has repeatedly said they are virtually defeated. His administration wants to show it is winning the fight against Boko Haram ahead of a presidential election in February at which he will seek a second term in office.